The present invention relates to a method for automatically limiting a pressure generated during operation in a hydraulic system when needed, said system being adapted to deliver a pressurized hydraulic fluid to at least one actuator adapted to perform a work function.
Below, the invention will be described in connection with a working machine in the form of a wheel loader. This is a preferred, but by no means limiting application of the invention. The invention can for example also be used for other types of working machines (or work vehicles), such as a backhoe loader, an excavator, or an agricultural machine such as a tractor.
A wheel loader can be utilised for a number of fields of activity, such as lifting and transportation of rock and gravel, loading pallets and logs. In each of these activities, different equipment is used, including implements in the form of a bucket, a fork implement and gripping arms. More particularly, the equipment comprises a load-arm unit, or boom, which is pivotally arranged relative to the wheel loader frame. Two actuators in the form of hydraulic cylinders are arranged between the frame and the load-arm unit in order to achieve a lifting and lowering movement of the load-arm unit. The implement is pivotally arranged on the load-arm unit. An additional actuator in the form of a hydraulic cylinder is arranged between the implement and the load-arm unit in order to achieve a tilting movement of the implement.
The hydraulic system comprises a pump adapted to supply the hydraulic cylinders with pressurized hydraulic fluid via a hydraulic circuit comprising a plurality of control valves.
As a rule, a wheel loader has more hydraulic functions than the above-mentioned lift and tilt function. Such additional hydraulic functions include steering, 3rd, 4th, and in some cases even more functions. Each function generally needs two shock valves, except lift which has one shock valve. For a machine with a 3rd and a 4th function, this implies nine shock valves.
Different functions require different flow rates. Furthermore, the same function requires different flow rates for piston and piston rod side. Machines of different sizes also have different flow rate requirements. In practice, only a few shock valves are used, where the one having the highest flow requirement decides the flow rate. This implies that most functions have unnecessarily large shock valves.
It is desirable to achieve a method which creates prerequisites for a more cost efficient system with maintained or improved service life.
A method according to an aspect of the present invention includes                detecting a pressure in at least one position in the system;—comparing the detected pressure value, or a value associated with the detected pressure value, with a first predetermined limit value; and        opening a flow communication between the actuator and a tank via a first conduit if the detected pressure value, or the value associated with the detected pressure value, exceeds the predetermined limit value.        
Thus, in this way, drainage to tank is actively controlled when needed. Preferably, at least one pressure sensor is adapted to detect the pressure to the respective function.
In this way, the limit value (for example the opening pressure) can be set as low as possible in all situations, which results in a smaller load on the constituent components.
According to a preferred embodiment, the flow communication is opened via a control valve being arranged on the first conduit and having the function to control the supply of the hydraulic fluid to and from, respectively, the actuator with the object of performing the work function. In case of an unexpected pressure increase, this control valve functions as a controlled shock valve. Preferably, separate inlet and outlet valves to the actuator are provided in order to control the function (for example a lifting and lowering movement).
According to another preferred embodiment, the method further comprises the step of opening a flow communication between the actuator and the tank via a second conduit via a shock valve. The shock valve is also called pressure limiting valve. The shock valve is preferably arranged in a conventional way as a passive (directly controlled by the pressure), for example spring-loaded, shock valve. By means of combining the opening of the control valve and the shock valve, drainage to tank at a desired rate can be obtained in case of a pressure shock.
Owing to the smaller size of the possibly included directly controlled shock valves and to fewer variants, a lower cost can be achieved. Furthermore, owing to the smaller directly controlled shock valves, the valve housing can be made smaller.
As a rule, the control valve opens more slowly than the shock valve, which in many cases implies that said flow communication between the actuator and the tank via the first conduit is opened after the shock valve has opened the flow communication between the actuator and the tank via the second conduit. In other words, the control valve is opened with a certain delay, so that the shock valve is opened first. It is possible, however, to ensure that the control valve opens substantially simultaneously as, or before the shock valve.
Preferably, a shock valve of a smaller size, i.e. with a lower nominal flow rate, than the electrically controlled outlet valve is used. The directly controlled shock valve, which is fast-acting, opens directly and flow drainage is initiated. Then, the electrically controlled control valve, which is capable of handling the larger flow requirement and draining it to tank, is opened.
According to another preferred embodiment, the method comprises the step of determining the flow rate to the tank on the basis of the detected pressure. In this way, the characteristics of the shock control function can be determined. The opening degree of the control valve is controlled, for example, on the basis of the pressure change in the actuator.
Further preferred embodiments of the invention and advantages associated therewith are apparent from the remaining claims and the following description.